For instance, a solid polymer fuel cell has a cell as a minimum unit, which is prepared by sandwiching an electrolyte membrane 52 made of a solid polymer film between two electrodes of a fuel electrode 50 and an air electrode 54 to form a membrane electrode assembly (MEA), and further sandwiching the MEA between two separators 40, as is shown in FIG. 4, and normally has a plurality of the cells stacked as a fuel cell stack (FC stack) to generate high voltage.
A mechanism of generating an electric power in a solid polymer fuel cell will be now described below. In general, a fuel gas, for instance a hydrogen-containing gas, is supplied to a fuel electrode (anode side electrode) 50. On the other hand, an oxidizer gas, for instance, a gas mainly containing oxygen (O2) or air, is supplied to an air electrode (cathode side electrode) 54. The hydrogen-containing gas is supplied to the fuel electrode 50 through a thin groove formed in the surface of a separator 40, and is decomposed into electrons and hydrogen ions (H+) by a catalytic action of the electrode. The electrons move to the air electrode 54 from the fuel electrode 50 through an external circuit to generate an electric current. On the other hand, the hydrogen ions (H+) pass through an electrolyte membrane 52 to reach the air electrode 54, are coupled with oxygen and the electrons which have passed through the external circuit, and produce reaction water (H2O). Heat generated simultaneously with a coupling reaction between hydrogen (H2), oxygen (O2) and the electrons is collected by cooling water. The water (hereinafter referred to as “reaction water”) produced in the cathode side at which the air electrode 54 exists is discharged from the cathode side.
In addition, the two separators which sandwich the above described MEA are partition plates which separate hydrogen gas from oxygen gas and simultaneously have a function of electrically connecting stacked cells in series. The two separators also have fine grooves on the surface to form an uneven shape thereon, which are a gas flow passage for passing the hydrogen-containing gas, the oxygen-containing gas and air therethrough.
An example of a structure of a conventional cell is shown in FIG. 5 and FIG. 6. FIG. 5 shows a cross section along the line A-A′ of FIG. 6.
As is shown in FIG. 5 and FIG. 6, supply communication holes 12a, 12b and 12c for supplying a fuel gas, an oxidizer gas and cooling water are provided respectively in one end of two sheets of separators 110 and 120, and discharge communication holes 14a, 14b and 14c for discharging the fuel gas, the oxidizer gas and the cooling water are provided in another end of the two sheets of the separators 110 and 120, with gas channels 152 and 154 for passing the fuel gas and the oxidizer gas respectively therein, which have been supplied from the supply communication holes 12a and 12b respectively, being further provided in the separators 110 and 120. In addition, recesses 106 and 116 are formed in facing surfaces of the separators 110 and 120 respectively, sealants 60a and 60b for respectively separating the fuel gas from the oxidizer gas are provided in periphery portions on both faces of an MEA 30 which is an assembly, and the sealants 60a and 60b are bonded to the two sheets of the separators 110 and 120 by adhesives 70a and 70b respectively to form the cell.
However, when a separator is made from stainless steel (so-called, SUS), a passive film 22 made of a chromium oxide film is formed on the surface of a SUS separator substrate 20, as shown in FIG. 3. On the other hand, an eco-friendly material tends to be used in any field, and in the above described adhesive and sealant, a water-soluble resin, for instance, tends to be used in place of a conventional lipophilic resin which is soluble in a solvent. However, the above described passive film 22 has low affinity with the hydrophilic water-soluble resin. Accordingly, when the above described water-soluble resin is directly bonded onto the SUS separator substrate 20 as the adhesive or as the sealant without the use of the adhesive, the water-soluble resin is occasionally peeled off due to a shear stress generated in a step of stacking cells for use in a fuel cell having the above described assembly sandwiched between a pair of the separators into a stack form, and fastening the stack by pressurizing the manifold, or is peeled off due to thermal expansion occurring during use, and is occasionally even attached and then detached, because the water-soluble resin has low adhesion to the substrate.
Technology is also proposed for the purpose of preventing the corrosion of a main body of a metal sheet having the manifold for passing a fuel gas into and out of a fuel gas channel in a central part in the separator of a solid polymer fuel cell, by forming a fluorine resin coating layer on an end surface of the above described manifold and protecting an exposed surface of the main body of the metal sheet with the fluorine resin coating layer (for instance, see Japanese Patent Laid-Open Publication No. 2002-25574).
In recent years, as demand for fuel cells has increased more and more, a fuel cell has been expected to have improved durability.